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International audience; Comprehension of the electronic properties of nano-objects is a key to defining dedicated properties, which can be adjusted by changing their size. Beyond confinement effects, the presence of interfaces, i.e., places where there is an abrupt change of electronic density, should also play a role. Time-dependent density functional theory (TD-DFT) is a state-of-the-art ab initio formalism in which this effect is accounted for through the so-called local field effects. In an earlier paper [S. Mazzei and C. Giorgetti, Phys. Rev. B 106, 035431 (2022)], we showed that the framework inherited from three-dimensional crystals could not provide reliable absorption spectra. In the present paper, we propose to calculate the macroscopic average of the dielectric tensor of a quasi-twodimensional (2D) object from the response function of the density to the total macroscopic potential in order to avoid use of the so-called Adler and Wiser formula. We evidence that the inclusion of interfaces in the thickness of the slab causes the response function for the out-of-plane component to move sharply from the bulk absorption resonance to the plasmon one. This shows that the longitudinal-longitudinal contraction of the dielectric tensor is no longer equal to the transverse-transverse one in a quasi-2D object for out-of-plane perturbation. Nevertheless, we also show that the macroscopic average of the dielectric tensor of an ultrathin slab calculated within the longitudinal formalism of TD-DFT depicts the properties of the transverse reflectance and transmittance spectra of a thin slab.

International audience; Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard AA′ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated h11BN flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of h11BN. Hence, we show that hBN presents an AA′ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.

International audience; Electrostatic gating or alkali metal evaporation can be successfully employed to tune the interface of layered black phosphorus (BP) from a semiconductor to a 2D Dirac semimetal. Although Angle Resolved Photoelectron Spectroscopy (ARPES) experiments have captured the collapse of the band gap in the inversion layer, a quantitative estimation of band structure evolution has been hindered by the short escape depth and matrix elements of the probed photoelectrons. Here, we precisely monitor the evolution of electronic states using time-resolved ARPES at a photon energy of 6.3 eV. The probing depth of laser-based ARPES is long enough to observe the buried electronic states originating from the valence band maximum. Our data shows that the band gap has a maximal value of 0.32 eV in the pristine sample, and that it shrinks down monotonically by increasing the carrier concentration in the topmost layer. Most interestingly, the band velocity of the valence band increases by a factor of two along the armchair direction, surpassing the value reported in graphene on silicon carbide (SiC). This control of band structure via external gating will be of interest with regard to the design of optoelectronic devices.

Manipulation of intrinsic electronic structures by electron or hole doping in a controlled manner in van der Waals layered materials is the key to control their electrical and optical properties. Two-dimensional indium selenide (InSe) semiconductor has attracted attention due to its direct band gap and ultrahigh mobility as a promising material for optoelectronic devices. In this work, we manipulate the electronic structure of InSe by in situ surface electron doping and obtain a significant band gap renormalization of ∼120 meV directly observed by high-resolution angle resolved photoemission spectroscopy. This moderate doping level (carrier concentration of 8.1 × 1012 cm-2) can be achieved by electrical gating in field effect transistors, demonstrating the potential to design of broad spectral response devices.

Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy (2PPE), we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Gamma point of the Brillouin zone (BZ). STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction band minimum (CBM) and a very sharp one near the maximum of the valence band (VMB). This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle resolved photoemission spectroscopy (ARPES) investigation. technologies. In fact, a hole effective mass of about m/m0 = -0.95 gammaK direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic

International audience; We investigate black phosphorus by time- and angle-resolved photoelectron spectroscopy. The electrons excited by 1.57 eV photons relax down to a conduction band minimum within 1 ps. Despite the low band gap value, no relevant amount of carrier multiplication could be detected at an excitation density 3–6 × 1019 cm–3. In the thermalized state, the band gap renormalization is negligible up to a photoexcitation density that fills the conduction band by 150 meV. Astonishingly, a Stark broadening of the valence band takes place at an early delay time. We argue that electrons and holes with a high excess energy lead to inhomogeneous screening of near surface fields. As a consequence, the chemical potential is no longer pinned in a narrow impurity band.

High-energy electronic excitations of graphene and ${\mathrm{MoS}}_{2}$ heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/${\mathrm{MoS}}_{2}$/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack.

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the $\overline{M}$ valley. Subsequently, the electronic cooling is dictated by Fr\"ohlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\overline{M}$ valley could travel through a multilayer flake of InSe with a lateral size of 1 $\ensuremath{\mu}\mathrm{m}$. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

More than 50 years after the first experimental observation of second-harmonic generation, the theoretical description of second-harmonic generation is still under debate, whereas it is well understood from an experimental point of view. This is the gap that this thesis aims to fill. This work aims to improve the theoretical description and understanding of the generation of second-harmonic from the surfaces of crystalline semiconductors. When applying an external electric field to a dielectric material, electric dipoles are created at a microscopic level. These dipoles are responsible for the apparition, inside the material, of an induced field. The fluctuations of the electric field at a microscopic level, the density fluctuations or any kind of microscopic inhomogeneities must be taken into account when describing the optical properties of a system. These effects are often referred as “local-field effects”. These local-field effects have been widely studied in the past and in particular their effects on the optical properties of bulk materials are now well established. In the case of surfaces, the theoretical description and the numerical simulations are more intricate than for bulk materials. The abrupt change in the electronic density leads to a huge variation of the electric field at the interface with vacuum. As a result, strong effects of the local-field are expected, in particular in the direction perpendicular to the plane of the surface. The goal of this thesis is to quantify how important these effects are for the linear and second-order optical properties of surfaces. A macroscopic theory of second-harmonic generation from crystal surfaces has been developed in order to account for local-field effects. The latter are calculated from first-principles, in the framework of the Time-Dependent Density-Functional Theory (TDDFT). The primary interest is the description of non-linear optical responses of surfaces, but new theoretical tools for improving the description of local-field effects in the case of linear optics have also been developed. The numerical simulations have been focused on the Si(001) surface, and the macroscopic formalism developed during this thesis has been applied to three surface reconstructions, namely the clean Si(001)2x1, the monohydride Si(001)2x1:H and the dihydride Si(001)1x1:2H surfaces. Comparison with available experimental results is also reported.

International audience; Low-dimensional materials differ from their bulk counterpart in many respects. In particular, thescreening of the Coulomb interaction is strongly reduced, which can have important consequencessuch as the significant increase of exciton binding energies. In bulk materials the binding energyis used as an indicator in optical spectra to distinguish different kinds of excitons, but this is notpossible in low-dimensional materials, where the binding energy is large and comparable in size forexcitons of very different localization. Here we demonstrate that the exciton band structure, whichcan be accessed experimentally, instead provides a powerful way to identify the exciton character.By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane andsingle-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensionalmaterials, highlighting the different role played by the exchange electron-hole interaction and by theelectronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

International audience; Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects. Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using the “effective-medium theory” with vacuum and also in terms of interaction between replicas, as the supercell approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) 2×1 surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to nonlinear optics for surfaces.

International audience; One of the big challenges of theoretical condensed-matter physics is the description, understanding, and prediction of the effects of the Coulomb interaction on materials properties. In electronic spectra, the Coulomb interaction causes a renormalization of energies and change of spectral weight. Most importantly, it can lead to new structures, often called satellites. These can be linked to the coupling of excitations, also termed dynamical effects. State-of-the-art methods in the framework of many-body perturbation theory, in particular, the widely used GW approximation, often fail to describe satellite spectra. Instead, approaches based on a picture of electron-boson coupling such as the cumulant expansion are promising for the description of plasmon satellites. In this work, we give a unified derivation of the GW approximation and the cumulant expansion for the one-body Green’s function. Using the example of bulk sodium, we compare the resulting spectral functions both in the valence and in the core region, and we discuss the dispersion of quasi-particles and satellites. We show that self-consistency is crucial to obtain meaningful results, in particular, at large binding energies. Very good agreement with experiment is obtained when the intrinsic spectral function is corrected for extrinsic and interference effects. Finally, we sketch how one can approach the problem in the case of the two-body Green’s function, and we discuss the cancellation of various dynamical effects that occur in that case.